The 90K protein of SV40 appears to be required for the maintenance of transformation since many cell lines transformed by viral strains containing a temperature-sensitive mutation in this protein (tsA mutants) are temperature-sensitive for their transformed phenotype. On the other hand many cell lines are not temperature-sensitive. We have examined the integration patterns of SV40 into the host in the temperature-sensitive and -insensitive lines. Although some correlation between copy number and temperature-sensitivity exists, there is not a simple correlation. In separate experiments we have shown in permissively infected monkey cells that SV40 replicates greater than 98 percent from its normal origin, approximately 1 percent from a second origin 180 degrees around the genome and possibly as a rare event from other sites. When tsA mutants are shifted to the nonpermissive temperature, replication from the normal sites is eliminated and initiation appears to start randomly on the SV40 genome.